The present invention relates to a blow-molding machine for fabricating a bottle-shaped container of biaxially oriented plastic material such as polyethylene terephthalate and, more particularly, to a jig used for the blow-molding machine.
When polyethylene terephthalate is biaxially oriented to fabricate a bottle-shaped container in an ordinary process for molding the container of biaxially oriented plastic material, it is maintained at a temperature suitable for orienting the material immediately before blow-molding the material. To this end, there has been previously proposed, in Japanese Patent Application No. 159,155/77 corresponding to U.S. Pat. Application Ser. No. 973,439, filed on Dec. 26, 1978, now U.S. Pat. No. 4,233,010, an apparatus for producing biaxially oriented plastic containers, which comprises a jig having a neck support for holding the neck of an injection-molded plastic piece of closed-end cylindrical shape to be formed into a bottle-shaped container, which piece will be hereinafter called the "piece". The piece is positioned upside down; a mandrel is formed beneath the neck support; and a core shaft for longitudinal orientation is provided inside the mandrel so as to be freely moved up or down and has a blow-molding air passage. The jig set with the piece is attached to a core metal which is moved in a heating chamber immediately before use of a blow-molding machine and is automatically operated while the jig is moved to substantially uniformly heat the piece by means of environmental infrared rays or far infrared rays.
However, when the piece is biaxially oriented to form larger bottle-shaped containers, it must be large and thick enough to form the large bottle-shaped container. As the piece is increased in size and thickness, it becomes difficult to uniformly heat the piece, particularly in its thickness direction, only by the external infrared ray or far infrared ray heating system. When the piece is abruptly heated in this case, it tends to be deformed, to form irregularly thick portions, or to retain its shrinkage to cause the bottle-shaped container thus blow-molded from the piece to be fabricated improperly. In order to avoid these drawbacks, it is necessary to gradually heat the piece sufficiently in uniform temperature distribution thereat.